From the answer:
Hi Ali, Let me share with you some information about QuickBooks Online training.... You can go over to this video tutorial for visual guide on how to work with QuickBooks Online: QuickBooks Tutorials.

From the answer:
When you make an advanced payment to a supplier, enter the transaction normally and use accounts payable/supplier as the expense (reason) for the payment then enter the bill, and in pay bills, select the bill and apply the supplier credit the payment